In an ink jet recording method, recording is carried out by jetting fine drops of ink using a variety of mechanisms so as to adhere to a recording paper, and thereby forming ink dots on the recording paper. Therefore, the recording method of ink jet type has advantages in that it is noiseless, can provide full-color prints with ease and enables high-speed printing, compared with the recording method of dot impact type. However, it also has a weak point that printed ink is hard to dry since the ink used for ink jet recording is usually water base ink.
Thus, the paper used in the ink jet recording method is required to have properties of (1) ensuring high-speed drying of ink, (2) providing prints of high optical density, (3) being free from overflowing and feathering of ink, (4) not causing undulation by the absorption of ink, and so on.
The undulation concerning the required property (4) cited above can be classified into two types, namely the undulation of the type called cockling which is caused immediately after printing by expansion of paper due to absorption of ink and the undulation of the other type which is caused by shrinkage of paper due to drying of ink upon standing after printing.
As a measure against the cockle generation, the methods of using base papers having excellent dimensional stability are disclosed (Japanese Tokkai Sho 62-95285 and Japanese Tokkai Hei 4-91901, wherein the term "Tokkai" as used herein means an "unexamined published patent application"). With respect to the undulation of the latter type, on the other hand, the cause thereof has not yet been cleared up, so that measures which have hitherto been taken are insufficient. However, it has been known that the undulation of the latter type became serious when ink easily permeated into a raw paper because of reduced coverage of the recording layer provided thereon or when the ink jet recording paper comprised of a raw paper having a light basis weight and had low stiffness to be liable to undulation.
Further, it has been known that, even when the cockling trouble was avoided by the use of a base paper having good dimensional stability as described in Japanese Tokkai sho 62-95285, the undulation due to standing after printing often showed a tendency to increase so far as not only the base paper had a light basis weight and low stiffness but also the recording layer had a low coverage rate. Conversely, the recording papers having liability to cockling have been known to hardly suffer from the undulation trouble when allowed to stand after printing.
The aforementioned knowledge means that whether or not the recording paper causes cockles immediately after printing depends largely upon the expansibility thereof, while the undulation due to standing after printing depends primarily upon the irreversible shrinkage factor of the recording paper. In general, there is a tendency that the irreversible shrinkage factor is high in a recording paper which has undergone the so-called tension drying treatment, or a drying treatment under a bound condition, to acquire low expansibility and excellent dimensional stability. Accordingly, the physical properties responsible for the cockling are incompatible with those for the undulation due to standing after printing.